Valve clearance tolerance

[daverclasper]

Hi. On a MG site it said that an engine had valve clearance specified at  15 tho" cold. A later specification was 13 tho" hot for the same engine.

So I assume heat expansion of valve train at the rocker/valve is about 2 tho".

I'm wondering if I can relate this to my Triumph. as I have rocker pad indents on some rockers and can't get a good reading with feelers.

I have been setting a good rocker with feelers and relating the feel of this by hand to adjust the other rockers (someone on club Triumph recommended this technique). My main concern with this, is I could be setting them a bit tight and valves could burn (I guess secondery to that the valve would be opening a bit earlier/closing later.  

So, would the MG info imply that that as long as there at least a 2 tho+ gap, then the valves will be closing fully on an MG and therefore maybe something similar on a Triumph?.

Or maybe I'm way off, with this theory?.

Any help great please.

Thanks Dave 

  

[JohnD]

If you must persist with indented rockers, then an SPQR, or ClickFit device, will adjust them correctly.    Certainly better than guessing.

 

[Pete Lewis]

If you get sleepless over a hot to cold tolerance   set it cold  then measure what you get when hot

Johns right  a guess is very variable   the devices he's mention use the pitch of the tappet thread to  measure the gap from being closed to whatever  the spec is  by a  known rotaion of the screw   

Pete

[daverclasper]

Thanks a lot.

Have just bought 18 second hand rockers for a fiver with a view to dressing some of the better ones (I seem to remember your not keen on that John!).

Then hopefully I won't be persisting for ever. Will certainly bear the above devices in mind (I don't suppose counting amount of adjustment turns for 10 thou", then doing this from closed gap would work, guess not as folk would do it).

Cheers, Dave 

 

[JohnD]

Yes, it would!  If you know the pitch of the adjuster screw on the rocker, which is 24 TPI (Threads Per Inch),  so  1000/24 thou per full turn, or 41.7.

You want 15 thou, so 41.7/15 th of a turn or 0.36,  a third, give-or-take.

The ClickAjust (not ClickFit) provides detents that click, 30 to a turn, so 10 clicks.

Simples!

John

[daverclasper]

Thanks John. I thought I'd would go to the bottom of the class for that one

Just one thing. Haynes book showing valve to rocker at 10 thou for Vitesse?.

Cheers, Dave 

 

Edited May 10, 2018 by daverclasper

[JohnD]

If I may, do the maths!

But OOOPS!   I did the maths wrong last time!  And it still gave a near right answer!

The calculation should be 15/41.7, which as it happens = 0.3597, or 0.36 of a turn!

For 10 thou it's 10/41.7 = 0.24 of a turn, a quarter,  or 7.5 clicks (30/4).  You choose, 7 or 8, because one click will be 1.4thou.

John

[daverclasper]

Great. Thanks for that.

Out of interest I will note how much of a turn it takes to close the gaps and see what my guessing is like.

Dave

[daverclasper]

Have adjusted by above method, though to get 10 thou, it only required backing the adjusting screws 30 degree turns (rather than 45 degrees/ quarter turn)?.

I confirmed this on a number of valves.

Cheers, Dave

[Pete Lewis]

Think we have said if fettling the pads leave the wear pad just remove the sides

Rockers are generally flame or induction hardened the pads are pretty tough but easily workable to improve feel entry

Pete